1. Field of the Invention
The present invention relates to a scattered-ray grid, particularly for medical radiography systems having an X-ray source that generates an X-ray beam with a center ray, the scattered-ray grid being of the type having a carrier with absorption elements, particularly in the form of lead elements, which are formed as spaced pins in rows which are spaced apart from one another.
2. Description of the Prior Art
In radiography, particularly in medical diagnostics, scattered-ray grids are frequently employed to attenuate the scattered radiation emanating from the examination subject relative to the effective radiation. The grids commonly used today consist of a sequence of lead lamellas which are layered in alternation with lamellas made of a carrier material. X-rays that strike in the layers of the lamellas are attenuated only by the carrier material. Oblique radiation, on the other hand, is more or less absorbed by the lead lamellas.
Since such lead lamellas generate unavoidable lines on the X-ray image, and furthermore the number of lines per centimeter is limited for technical reasons having to do with production, German OS 197 29 596, corresponding to U.S. Pat. No. 6,047,044, discloses using spaced pins made of lead or some other absorptive material instead of the lead lamellas.
Owing to the conical beam geometry of the X-radiation which is common in projection radiography, the lead lamellas--and this applies accordingly to the parallel pins that serve in their stead--must not be oriented in parallel fashion. Rather, they must be directed in such a way that they are all oriented to the focus of the X-ray tube. This requirement means a significant outlay for production. Furthermore, the focusing of the scattered-ray grid is calculated only for a specific interval between focus and grid. When this interval is changed, the orientation conditions are no longer correct for the peripheral radiation, and clearly visible shadowing occurs at the image margins. Previously, the expense for producing focused grids was accepted as unavoidable, and beyond this several grids were employed, which had to be exchanged according to the selected focus-grid interval. But this was a significant disadvantage in terms of both production and handling, which was associated with significant added costs.
European Application 0 333 276 describes a scattered-ray grid for compensating vignetting which is provided with perforations that are arranged circularly, the density of which is not constant over the area.